Papers by Nikita Medvedev
In many solids, swift heavy ions (MeV-GeV) produce long cylindrical damage trails with diameters ... more In many solids, swift heavy ions (MeV-GeV) produce long cylindrical damage trails with diameters of a few nm. The projectiles induce excitation and ionization processes, releasing electrons which spread their kinetic energy via collisions with secondary electrons radially to the ion trajectory. The energy density in this cylindrical zone is determined by the ion energy deposited per unit path length (dE/dx) and by the range of the electron cascades. Since this range is directly connected to the maximum energy transfer to the electrons, the kinetic energy of the projectiles comes into play. For a fixed dE/dx value, the resulting energy density is lower for higher ion velocities. This so-called velocity effect was evidenced for latent tracks [1] as well as for etched tracks [2], revealing that track radii increase with decreasing ion velocity.
Physical Review B, 2010
A Monte-Carlo approach was applied for simulations of the early stage (first tens of fs) of kinet... more A Monte-Carlo approach was applied for simulations of the early stage (first tens of fs) of kinetics of the electronic subsystem of silica (SiO 2 ) in tracks of swift heavy ions (SHI) decelerated in the electronic stopping regime. At the first step multiple ionizations of target atoms by a projectile (Ca +19 , E = 11.4 MeV/amu) were described that gave the initial spatial distributions of free electrons having different momenta as well as distributions of holes in different atomic shells.
Time-resolved observation of band-gap shrinking and electron-lattice thermalization within X-ray excited gallium arsenide
Scientific Reports, 2015
Femtosecond X-ray irradiation of solids excites energetic photoelectrons that thermalize on a tim... more Femtosecond X-ray irradiation of solids excites energetic photoelectrons that thermalize on a timescale of a few hundred femtoseconds. The thermalized electrons exchange energy with the lattice and heat it up. Experiments with X-ray free-electron lasers have unveiled so far the details of the electronic thermalization. In this work we show that the data on transient optical reflectivity measured in GaAs irradiated with femtosecond X-ray pulses can be used to follow electron-lattice relaxation up to a few tens of picoseconds. With a dedicated theoretical framework, we explain the so far unexplained reflectivity overshooting as a result of band-gap shrinking. We also obtain predictions for a timescale of electron-lattice thermalization, initiated by conduction band electrons in the temperature regime of a few eVs. The conduction and valence band carriers were then strongly non-isothermal. The presented scheme is of general applicability and can stimulate further studies of relaxation within X-ray excited narrow band-gap semiconductors.
Journal of Physics D: Applied Physics, 2015
The event-by-event Monte Carlo model, TREKIS, was developed to describe excitation of the electro... more The event-by-event Monte Carlo model, TREKIS, was developed to describe excitation of the electron subsystems of various solids by a penetrating swift heavy ion (SHI), spatial spreading of generated fast electrons, and secondary electrons and holes cascades. The complex dielectric function formalism is used to obtain relevant cross sections. This allows to recognize fundamental effects resulting from collective response of the electron subsystem of a target to excitation that is not possible within the binary collisions approximation of these cross sections, e.g. differences in the electronic stopping of an ion and in the electron mean free paths for different structures (phases) of a material. A systematic study performed with this model for different materials (insulators, semiconductors, metals) revealed effects which may be important for an ion track: e.g. appearance of a second front of the excess electronic energy propagation outwards the track core following after the primary front of spreading of generated electrons. We also analyze how initial ballistic spatial spreading of fast electrons generated in a track turns to their diffusion at times ~10 fs after the ion passage. Detailed time-resolved simulations of kinetics of the electronic subsystem helped to understand the reasons of enhanced silicon resistance to SHI irradiation in contrast to easily produced damage in this material by femtosecond laser pulses. We demonstrate that fast spreading of excited electron from the track core on a sub-100 fs timescales prevents Si lattice from nonthermal melting in a relaxing SHI track.
Based on the Coulomb spike model, track formation depends strongly on the electrical resistivity ... more Based on the Coulomb spike model, track formation depends strongly on the electrical resistivity of a material, and ion tracks form only in insulating materials. However, there are no systemic studies of the effect of resistivity on the track formation in materials, such as SrTiO3 (STO), where with the addition of low concentrations of Nb, the resistivity dramatically decreases covering the entire electronic regime from an insulating to conducting material. In this study, high energy (8.6 MeV/u) ion-induced track formation in STO was characterized by transmission electron microscopy (TEM) and small-angle x-ray scattering (SAXS) techniques as a function of Nb-doping concentrations. Contrary to the Coulomb spike model’s predictions, the Nb-doping had no evident influence on track formation, as confirmed by both TEM and SAXS. This may be the result of the low electron density in the bulk material or the minor effect of the Nb-doping on the bonding in the material. In situ TEM studies of low energy (1 MeV Kr2+) ion irradiations show that the low concentration doping has a minor influence on the crystalline-to-amorphous transformation as a result of subtle structural variations of incorporated impurity atoms.
In many dielectrics, swift heavy ions of MeV-GeV energy produce tracks of a few nm in diameter. T... more In many dielectrics, swift heavy ions of MeV-GeV energy produce tracks of a few nm in diameter. The track morphology depends on energy loss (dE/dx), particularly the formation of a continuous damage trail requires a critical (dE/dx) c . Below this threshold, chemical etching of discontinuous tracks exhibits inhomogeneous pore sizes and the track etching efficiency drops below unity.
<title>Dynamic of electronic subsystem of semiconductors excited with an ultrashort VUV laser pulse</title>
Damage to VUV, EUV, and X-Ray Optics II, 2009
We investigate theoretically the interaction of a semiconductor with an ultrashort high-intensity... more We investigate theoretically the interaction of a semiconductor with an ultrashort high-intensity VUV laser pulse produced by new light source FLASH at DESY in Hamburg. Applying numerical simulations of excitations and ionization of electronic subsystem within a solid silicon target, irradiated with femtosecond laser pulse (25 fs, photon energy of 38 eV), the transient distribution of electrons within conduction band is obtained. The Monte Carlo method (ATMC) was extended in order to take into account the electronic band structure and Pauli's principle for electrons excited into the conduction band. Secondary excitation and ionization processes were included and simulated event by event as well. In the presented work the temporal distribution of the density of excited and ionized electrons, the energy of these electrons and their energy distribution function were calculated. It is demonstrated that due to the fact that part of the energy is spent to overcome ionization potentials, the final kinetic energy of free electrons is much less than the total energy provided by the laser pulse. We introduce the concept of an 'effective energy gap' for collective electronic excitation, which can be applied to estimate the free electron density after high-intensity VUV laser pulse. The effective energy gap depends on properties of the material as well as on the laser pulse.
Nature Photonics, 2013
Recently, few-femtosecond pulses have become available at hard X-ray free-electron lasers. Couple... more Recently, few-femtosecond pulses have become available at hard X-ray free-electron lasers. Coupled with the available sub-10 fs optical pulses, investigations into few-femtosecond dynamics are not far off. However, achieving sufficient synchronization between optical lasers and X-ray pulses continues to be challenging. We report a 'measure-and-sort' approach, which achieves sub-10 fs root-mean-squared (r.m.s.) error measurement at hard X-ray FELs, far beyond the 100-200 fs r.m.s. jitter limitations. This timing diagnostic, now routinely available at the Linac Coherent Light Source (LCLS), is based on ultrafast free-carrier generation in optically transparent materials. Correlation between two independent measurements enables unambiguous demonstration of ∼6 fs r.m.s. error in reporting the optical/X-ray delay, with single shot error suggesting the possibility of reaching few-femtosecond resolution.
Temporally resolved track creation in dielectrics after swift heavy ion irradiation. Part I: Monte-Carlo simulation
Temporally resolved track creation in dielectrics after swift heavy ion irradiation. Part II: Two temperature model
Transitions in matter triggered by intense ultrashort X-ray pulses
Three-zone model of track creation after swift heavy ion impact on insulators
Electronic Dynamic of Semiconductors after Ultra-short VUV Free-Electron Laser Pulse Irradiation
Electron Hole Plasma in Solids Induced by Ultrashort XUV Laser Pulses
Spatial and temporal resolved microscopic processes in dielectrics irradiated with swift heavy ions
Ultrafast Electronic and Atomic Kinetics in Solids After Femtosecond FEL Irradiation
Ultrafast electron kinetics in SiO2 under X-ray femtosecond irradiation
Excitation and relaxation of the electronic subsystem in solids after high energy deposition
ABSTRACT The full text is available here: https://kluedo.ub.uni-kl.de/files/2764/Medvedev_PhD_the... more ABSTRACT The full text is available here: https://kluedo.ub.uni-kl.de/files/2764/Medvedev_PhD_thesis.pdf Abstract: The present dissertation contains the theoretical studies performed on the topic of a high energy deposition in matter. The work focuses on electronic excitation and relaxation processes on ultrafast timescales. Energy deposition by means of intense ultrashort (femtosecond) laser pulses or by means of swift heavy ions irradiation have a certain similarities: the final observable material modifications result from a number of processes on different timescales. First, the electronic excitation by photoabsorption or by ion impact takes place on subfemtosecond timescales. Then these excited electrons propagate and redistribute their energy interacting among themselves and exciting secondary generations of electrons. This typically takes place on femtosecond timescales. On the order of tens to hundreds femtoseconds the excited electrons are usually thermalized. The energy exchange with the lattice atoms lasts up to tens of picoseconds. The lattice temperature can reach melting point; then the material cools down and recrystalizes, forming the final modified nanostructures, which are observed experimentally. The processes on each previous step form the initial conditions for the following step. Thus, to describe the final phase transition and formation of nanostructures, one has to start from the very beginning and follow through all the steps. The present work focuses on the early stages of the energy dissipation after its deposition, taking place in the electronic subsystems of excited materials. Different models applicable for different excitation mechanisms will be presented: in the thesis I will start from the description of high energy excitation (electron energies of ∼ keV), then I shall focus on excitations to intermediate energies of electrons (∼ 100 eV), and finally coming down to a few eV electron excitations (visible light). The results will be compared with experimental observations. For the high energy material excitation assumed to be caused by irradiation with swift heavy ions, the classical Asymptotical Trajectory Monte-Carlo (ATMC) is applied to describe the excitation of electrons by the impact of the projectile, the initial kinetics of electrons, secondary electron creation and Auger-redistribution of holes. I first simulate the early stage (first tens of fs) of kinetics of the electronic subsystem (in silica target, SiO2) in tracks of ions decelerated in the electronic stopping regime. It will be shown that the well pronounced front of excitation in the electronic and ionic subsystems is formed due to the propagation of electrons, which cannot be described by models based on diffusion mechanisms (e.g. parabolic equations of heat diffusion). On later timescales, the thermalization time of electrons can be estimated as a time when the particle- and the energy propagation turns from the ballistic to the diffusive one. As soon as the electrons are thermalized, one can apply the Two Temperature Model. It will be demonstrated how to combine the MC output with the two temperature model. The results of this combination demonstrate that secondary ionizations play a very important role for the track formation process, leading to energy stored in the hole subsystem. This energy storage causes a significant delay of heating and prolongs the timescales of lattice modifications up to tens of picoseconds. For intermediate energies of excitation (XUV-VUV laser pulse excitation of materials) I applied the Monte-Carlo simulation, modified where necessary and extended in order to take into account the electronic band structure and Pauli&#39;s principle for electrons within the conduction band. I apply the new method for semiconductors and for metals on examples of solid silicon and aluminum, respectively. It will be demonstrated that for the case of semiconductors the final kinetic energy of free electrons is much less than the total energy provided by the laser pulse, due to the energy spent to overcome ionization potentials. It was found that the final total number of electrons excited by a single photon is significantly less than ℏω/Egap. The concept of an &#39;effective energy gap&#39; is introduced for collective electronic excitation, which can be applied to estimate the free electron density after high-intensity VUV laser pulse irradiation. For metals, experimentally observed spectra of emitted photons from irradiated aluminum can be explained well with our results. At the characteristic time of a photon emission due to radiative decay of L−shell hole (t&lt;60 fs), the distribution function of the electrons is not yet fully thermalized. This distribution consists of two main branches: low energy distribution as a distorted Fermi-distribution, and a long high energy tail. Therefore, the experimentally observed spectra demonstrate two different branches of results: the one observed with…
Thermal and nonthermal melting of silicon exposed to femtosecond pulses of X-ray irradiation
Damage to VUV, EUV, and X-ray Optics V, 2015
Excitation and relaxation of the electronic subsystem of solids in nanosize tracks of swift heavy... more Excitation and relaxation of the electronic subsystem of solids in nanosize tracks of swift heavy ions (SHI, Е > 1 MeV/nucl, M ≥ 20m p ) are well separated by the following temporal stages: (a) at 10 -17 -10 -16 s, primary ionization of target atoms occurs in the vicinity of the projectile trajectory resulting in the first generation of fast delta-electrons. (b) Spatial spreading of these electrons and ionization of the next generation of δ-electrons take place in the time period of ~ 10 -16 -10 -14 sec. (c) Recombination of electronic vacancies and energy redistribution via radiation occurs on the time scale ≥ 10 -14 sec . Electrons also interact with ionic subsystem of target at all these stages.
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Papers by Nikita Medvedev